Many of those redirect access pages are not supported by the Opera browser. First disconnect from any networks that have a PRE to the right of the network name. Then, try this. Connect to the wifi network, confirm you have an IP address, and then touch the "Update" option on the start page menu. This should prompt a secondary browser to pop up so you can agree. Then you should be able to update as well as browse with Opera.
This does not work with all redirect networks, but it does with most that we've tested in hotels, restaurants, etc. I hope this helps.